(1) Field of the Invention
The present invention relates to rechargeable power sources for portable electronic devices such as camcorders, cell phones, laptop computers and toys, and more particularly to positive electrode-active materials for lithium, lithium-ion and lithium-ion polymer batteries and methods of making and using such materials.
(2) Description of the Related Art
Rapid technological developments in the electronics and computer industry have created a large consumer market for a variety of batteries. Today, batteries are used to power almost every portable electronic device, such as cell phones, laptop computers, camcorders, portable radios, cameras and toys. With the continuing miniaturization in the electronic industry and in portable electronic devices, the demand for lightweight, compact, and yet high-energy density batteries has been steadily increasing. In addition, a need for more efficient utilization of the available energy resources as well as air-quality-control has generated an enormous interest in the development of advanced high energy density batteries for electric powered vehicles. Furthermore, cost effectiveness, rechargeability, and better safety characteristics have been other factors driving the battery market.
Lithium-ion and lithium-ion polymer batteries represent a new generation of lightweight, compact, and yet high-energy power sources. This is particularly true for lithium-ion polymer cells since they can be made very thin, and with great shape flexibility. Lithium-based batteries are attractive for energy storage because of lithium's high specific capacity (3800 ah/kg) and low electronegativity (0.97). These properties lead to energy cells (“cells”) with high energy density and high voltage. The materials that are used to produce lithium-based batteries are also less toxic than the components of nickel cadmium or lead acid cells, and their disposal poses fewer environmental problems.
The commercial and military applications of lithium-based batteries date back to the 1960's and 1970's. Primary lithium batteries (single use, lithium metal as anode) were commercialized in the 1970's. These were followed by the development of rechargeable secondary cells that also used lithium metal as anodes in the early 1980's.
Typically, a lithium cell has been made up of a lithium metal negative electrode (“anode”), a positive electrode (“cathode”), such as manganese oxide (Mn2O4), and some type of an electrolyte that serves as an ionic path for lithium ion between two electrodes. During discharge, lithium ions from the metallic anode pass through the electrolyte to the electrochemical materials of the cathode whereupon they release electrical energy to an external circuit.
Since their commercialization, primary lithium cells (that is, cells which are used as a power source for one application and then are discarded) have been widely used in both commercial and military applications, while most rechargeable secondary cells have been struggling on the market. Difficulties associated with secondary cells stem from reactions of lithium metal with electrolytes and the changes in the lithium surface that occur after repetitive charge-discharge cycling. Furthermore, the high reactivity of the lithium metal presents a fire and explosive hazard, which becomes a serious concern when use is considered in larger cells.
In addressing the issues associated with highly reactive and irreversible metallic lithium anodes, a more advanced and inherently safer approach, the so-called rocking chair or lithium-ion cell, was adopted in the late 1970's and early 1980's. In this approach, a lithium metal negative electrode is replaced by a lithium intercalation material or compound, such as lithiated carbon or lithiated metal oxides, while another lithium intercalation material is used for the positive electrode, or cathode. The operation of such a system involves the shuttling of lithium ions back and forth between the two intercalation compounds during charge/discharge cycles. The output voltage of these types of rocking chair cells is determined by the difference between the electrochemical potential of lithium within the two lithium intercalating electrodes.
An insertion compound is a host into/from which a guest species may be topotactically and reversibly inserted/extracted over a finite range of solid solution. Once such example would be graphite, which is known to reversibly intercalate lithium-ions and has been used as an anode material in lithium-ion batteries. Further examples of such compounds are lithium metal oxides, where the metal can be selected from a wide range of metals.
Research and commercial development concerning rocking chair batteries has been extensive since the adoption of that product. The first commercial lithium-ion cell based on the carbon anode and LiCoO2 was marketed by Sony Corporation in about 1990.
Positive electrodes (cathodes) are the most critical component in the lithium-ion and lithium-ion polymer batteries, as they determine the battery performance attributes such as operating voltage, energy density, and cycle life. For the purposes of this specification, the term “operating voltage” shall mean that working voltage produced when the battery is fully operational. For the purposes of this specification, the term “energy density” shall mean the energy produced per unit volume and or weight. For the purposes of this specification, the term “cycle life” shall mean the number of cycles that the battery can experience in its effective lifetime. In this regard, lithium insertion compounds as cathode materials for lithium-ion batteries have been extensively investigated in the past two decades. The electrochemical potential range of lithium insertion compounds (with respect to the Li metal) for a wide variety of compounds has been obtained and documented such as in Manthiram et al, JOM, 49: 43 (1997).
Among the insertion compounds that have been evaluated, LiCoO2, LiNiO2, and LiMn2O4 have been found to be most attractive. The theoretical capacities of both LiNiO2 and LiCoO2 are about 275 Ah/kg. However (from a practical matter), only a fraction of the theoretical capacity can be reached. Compared to LiNiO2 and LiCoO2, LiMn2O4 gives a lower theoretical capacity of 148 Ah/kg and typically no more than 120 Ah/kg can be obtained. At present, most commercial lithium-ion batteries use LiCoO2 as the cathode material, whereas LiNiO2 and LiMn2O4 are much less common.
The preference of LiCoO2 in commercial cells stems from its better cycleability over LiNiO2 and LiMn2O4, despite the fact that LiCoO2 is the most expensive of the three compounds. The reversible capacity of LiNiO2 is limited by irreversible phase transition on first delithiation, in which more than 10% of initial capacity can be lost. In addition, the thermal stability of LiNiO2 is not good at its delithiated state, which can lead to safety concerns because of gaseous oxygen release. LiMn2O4, on the other hand, experiences problems due to Mn dissolution from electrodes into electrolyte solution at high discharge rate, Jahn-Teller effects at the end of the deep discharge, and parasitic phase formation during the charge/discharge cycles. For further information in this regard see Thackeray, M., et al., Electrochemical and Solid State Letters, 1:7-9 (1998).
Despite the tremendous effort employed in improving the performance of each type of insertion compound by different preparation procedures, the charge/discharge properties of these compounds are still not sufficient to satisfy commercial requirements. At present, at least, a single metal-based cathode material cannot meet all of the performance requirements of lithium-ion batteries. Accordingly, the recent trend in battery development has been shifted to multi-metallic insertion compounds that can take advantage of the attributes of each metal component. See for example: Huang D. Advanced Battery Technology, p. 21, Nov. (1998).
For instance, in Cedar et al., Nature, 392:694 (1998), it has been shown that part of the transition metal in a cathode material could be replaced by other elements such as non-transition-metal ions, while still retaining electrochemical Li-activity at higher voltage. The article suggested that oxygen atoms are playing an important role in promoting the electron exchange and the cell voltage correlates with increased oxygen participation. Cedar and coworkers apparently observed improved cell voltage and better cycleability in Al-adopted bimetallic LixAlyCo1-yO2 and LixAlyMn1-yO2 systems. See, also, Cedar et al., Computational Materials Science, 161:8 (1997), and Jang et al., Electrochemical and Solid State Letters, 13:1 (1998).
Furthermore, U.S. Pat. No. 5,370,948 to Hasegawa et al., U.S. Pat. No. 5,264,201, to Dahn et al., U.S. Pat. No. 5,626,635 to Yamamura et al., as well as academic publications by Zhong et al., in J. Electrochem. Soc., 144: 205 (1997); Amine et al., in J. Power Sources, 68: 604 (1997), Fey et al. in J. Electrochem. Soc., 141: 2279 (1994); Sigala et al., in Solid State Ionics, 81:167 (1995)); and Ein-Eli et al., J. Electrochem. Soc., 145:1238 (1998), describe binary cathode materials. Liu et al., in J. Electrochem. Soc., 879:143 (1996), describe the production of composite oxides of one or two metals by forming a polymeric resin throughout which metal ions are distributed. They show that the resin is homogeneous at an atomic level and can be calcined at temperatures that are lower than normally used to yield composite oxides that have high surface area and unique morphologies.
Ternary and quaternary cathode combinations have also been explored, albeit much less than binary systems. In this regard, U.S. Pat. Nos. 5,783,333 and 5,795,558, to PolyStor Corporation (Dublin, Calif.) and Japan Storage Battery Co., Ltd. (Tokyo, Japan), respectively, as well as academic publications by Ein-Eli et al. in J. Electrochem. Soc., 146:908 (1999) and Gao et al., in Electrochem. & Solid State Letters, 1:117 (1998), describe such systems.
U.S. Pat. Nos. 5,718,989 and 5,795,558 to Aoki et al., describe positive electrode-active materials for a lithium secondary battery and a method of producing them. The cathode materials described include formulations such as LiNi1-x-y-zCoxMnyAlzO2, but cobalt content never exceeds 25 mol percent, manganese content never exceeds 30 mol percent, and aluminum content never exceeds 15 mol percent of the combined Ni, Co, Mn and Al content. These materials appear to be produced by a process which does not start with a homogeneous solution of the four metals that make up the composite oxide. The process, therefore, would not be expected to provide molecular level mixing of all four of the metals before calcination. Moreover, the method appears not to use low covalent Mn (II) salt as the source of manganese, and would, therefore, not be expected to provide efficient oxidation of the mixture at lower temperatures, so as to avoid phase separation of the calcined composite oxide material. Electrodes that are produced by the disclosed process were apparently never tested by charging to over 4.1 volts, too low a voltage level to provide any indication of the performance of such materials at higher voltage levels, i.e., above about 4.6 volts.
U.S. Pat. Nos. 5,783,333 and 6,007,947 to Mayer disclose the formation of ternary material formulated as LixNiyCoxMzO2 and suggest that quaternary combinations are possible.
Despite these advances, there is still a need for a new generation of cathode-active compounds that can provide high capacity with low cost, good cycleability, and high stability, particularly at voltage levels above about 4.2 volts. There is also a need for methodologies for preparing homogeneously mixed multi-metallic compositions that can effectively combine each metal's performance characteristics. In addition, there is a need to find such cathode-active compounds that minimize the irreversible capacity loss during the first and subsequent delithiation cycles and that have increased mid-point cell voltage. It is to such needs that the present invention is directed.